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Mushroom Body is a Quadruple Structure - Figure 3
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Each clone of the four mushroom body neuroblasts (MBNbs) contributes
to the entire mushroom body structure.
Click each thumbnail image to see the original figure.
(A-C) A larval MBNb clone induced just after hatching with the FRT-GAL4
system (Fig. 2) and stained at the end of the larval
stage. (A) Frontal view of the anterior MB region,
(B) horizontal view of the entire MB, and (C)
frontal view of the posterior MB region.
At the top of a tightly packed cluster of smaller Kenyon cells (KC), a single
neuroblast (MBNb) is observed. The calyx (ca) is seen below the cell cluster.
At the root of the pedunculus, the fibres of all the cells form a single
fascicle. The bundle de-fasciculates to form medial (M) and lateral (L)
bundles after running one-third the length of the pedunculus. The medial
bundle bifurcates at the frontal end of the pedunculus to form alpha and
beta lobes. The lateral fibres form the spur (sp) structure at the frontal
end and bend inwards to the gamma lobe. Some of the lateral fibres bend
upwards to form the alpha' lobe, which is located adjacent to the alpha
lobe.
The cells of a single clone apparently contribute to all the known MB substructures.
It is also clear that the progeny of a MBNb innervate exclusively structures
within the MB neuropile.
(D-F) An adult MBNb clone (induced at the end of the second instar,
stained on the day of eclosion). (D) Frontal view
of the anterior MB region, (E) horizontal view
of the entire MB, and (F) frontal view of the posterior
MB region.
The structure of the clones after pupal metamorphosis is surprisingly unchanged.
The cell bodies are still tightly packed, and the fasciculation of the fibres
persists even in the adult brain. As in larvae, a single clone contributes
to the entire MB structure. Aside from their relative position in the cortex,
no differences were observed among the MBNb clones.
While the fibres in the medial bundle (M) are tightly bound, those in the
lateral bundle (L) show a more diffuse structure (E).
A small subset of the lateral fibres run dorsal, ventral, or even medial
to the medial bundle before they project to the spur region.
Note the irregular-shaped glial cells (CBG) in the cell body cluster of
the Kenyon cells in (F). This suggests that the
MBNbs give birth not only to neuronal Kenyon cells but also to the cell
body glial cells.
(G-I) An adult MBNb clone labelled at the mid-pupal stage. (G) A brain without any MBNb clones, (H) with a clone, and (I) the frontal view of the posterior region of the clone.
The four MBNbs are the only neuroblasts that continue proliferating until
very late in the pupal stage (Ito and Hotta, 1992). When the heat shock
is given at mid pupal stage (H), a tightly packed
MB cell cluster near the brain surface is labelled. The fibres form a single
medial pedunculus bundle towards the alpha and beta lobes. Note that a much smaller number of cells are labelled than in the clones induced at earlier stages (compare I with C and
F).
Thus, it is very likely that most of the Kenyon cells in the late-induced
clones innervate only the medial pedunculus bundle and the alpha and beta
lobes but not the lateral bundle and the gamma lobe.
All figures are montaged from 20-50 photographs. Bar, 50 um (10-um scale).
Note
In Fig. H, a few scattered MB neurones that project
through the lateral bundle are also labelled. As shown in Fig.
G, these scattered cells are labelled also in brains that do not contain
any MBNb clone. Thus we infer that these cells were labelled either by postmitotic
Flippase activity or by the division of the ganglion mother cells.
Reference
Ito, K., and Hotta, Y. (1992). Proliferation pattern of postembryonic neuroblasts
in the brain of Drosophila melanogaster. Dev. Biol. 149, 134-148.
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